This invention relates to the preparation of epoxy resins. In one aspect, the invention relates to deactivation of a phosphonium salt catalyst. In a further aspect, the invention relates to terminating an epoxy fusion reaction in which a polyepoxide is reacted with a phenol or other acidic compound in the presence of a phosphonium salt catalyst.
It is known to prepare epoxy resins by reacting a compound having a vicinal epoxide group, such as epichlorohydrin, with bisphenol-A in the presence of sodium hydroxide. To prepare linear, high molecular weight epoxy resins, it is common practice to first prepare an epoxy resin having a relatively low weight per epoxide and then react it, in the presence of a catalytic amount of a hydrocarbyl phosphonium halide, with a compound containing phenolic hydroxyl groups. The use of phosphonium halide catalysts for such an epoxy "fusion" or "advancement" reaction results in the formation of linear, high molecular weight epoxy resin.
In order to control the molecular weight of this epoxy resin reaction product, it is necessary that the catalyst be deactivated at the desired phase of the reaction. How readily the fusion catalyst is deactivated depends upon the particular catalyst and the conditions under which the reaction is being carried out. For example, certain phosphonium halide catalysts such as ethyl triphenyl phosphonium iodide are subject to attack by hydroxyl ions. Performing the advancement reaction in a reaction mixture which contains water results in deactivation of the catalyst as the reaction proceeds. This rather slow catalyst deactivation can be used to advantage to control the speed of the advancement reaction, preventing too-rapid reaction as the desired molecular weight is approached.
By contrast, certain hydrocarbyl phosphonium salt advancement catalysts are very stable to hydroxyl ion attack. Such stable catalysts, such as methylene bis(triphenyl phosphonium bromide), for example, are not deactivated to an appreciable extent by residual water in the reaction mixture. If the reactivity of the catalyst is not controlled, the molecular weight of the fusion product will continue to increase. This can result in difficulty controlling the properties of the fusion resin and changes in properties during storage of the resin.
It would thus be desirable to provide a means whereby hydrocarbyl phosphonium salf fusion catalysts, including those which are highly resistant to hydroxyl ions, can be deactivated in the epoxy fusion reaction medium at the desired point of the fusion process.